1. Field of the Invention
The present invention relates to a magnet plug mounted in a gear box or the like to adsorb and remove iron powder or the like mixed in the oil within the gear box.
2. Description of the Prior Art
A magnet plug is constructed by fitting and fixedly securing a permanent magnet in and to a recess formed in a tip end surface of a plug main body to be threadedly mounted to a gear box or the like.
In the prior art, a magnet plug constructed by shrivelling a tip end edge of a plug main body against a permanent magnet piece fitted in a recess of the main body when the permanent magnet piece is fixedly secured to the plug main body, was proposed.
One example of such type of magnet plugs in the prior art is shown in FIGS. 6 to 8. With reference to these figures, a bottom end of a permanent magnet piece (c) is fitted in a recess (b) formed in a top end portion of a plug main body (a), and the magnet plug is completed by shrivelling a tip end edge of the plug main body in a spot-like manner into a stepped portion at the bottom of the permanent magnet piece (c) with shrivelling jigs applied directly to a number of locations of the periphery at the tip end of a threaded portion of the plug main body (a), as indicated by reference character (d).
In addition, another example of such type of magnet plugs in the prior art is shown in FIGS. 9 to 11. In the case of this construction, a tip end edge of a plug main body (a) is shrivelled into straight grooves (e) formed in the axial direction on the diametrically opposite outer circumferential surface portions of a permanent magnet piece (c) fitted in a recess (b) formed in a top end portion of the plug main body (a) by applying shrivelling jigs at two locations on the tip end edge of the threaded portion of the plug main body (a), as indicated by reference character (d').
In the above-described magnet plugs in the prior art, since the permanent magnet is fixed to the plug main body by spot-shrivelling, looseness between the plug main body and the permanent magnet piece is inevitable, and also as a shrivelling jig is applied directly to the threaded portion of the plug main body, distortion would be generated in the threaded portion by shrivelling, and in the worst case the plug main body would become unable to be screwed into a threaded bore in the gear case or the like.
Furthermore, as the bottom portion of the permanent magnet piece is directly fitted in the threaded portion of the plug main body, the threaded portion would be magnetized, and so, there exists a problem that the threaded portion of the plug as well as the threaded bore for mounting in the gear box or the like may be damaged by iron powder or the like adsorbed by the magnetized threaded portion.
It is therefore one object of the present invention to provide an improved magnet plug in which a permanent magnet piece can be firmly secured to a plug main body without damaging a threaded portion of the plug main body nor without magnetizing the plug main body.
According to one feature of the present invention, there is provided a magnet plug comprising a plug main body including a threaded portion, a hollow cylindrical piece projected from the tip end surface of the threaded portion of the plug main body and having a smaller outer diameter than that of the threaded portion, and a permanent magnet piece having an annular step formed on its outer peripheral surface with its bottom portion fitted in the hollow cylindrical piece and fixedly secured thereto by shrivelling the entire peripheral wall of the hollow cylindrical piece into the annular step.
According to the present invention, as described above, a hollow cylindrical piece having a smaller outer diameter than that of a threaded portion of a plug main body is projected from a tip end surface of the threaded portion of the plug main body, a bottom portion of a permanent magnet piece is fitted in the hollow cylindrical piece, then the entire peripheral wall of the hollow cylindrical piece is shrivelled into the annular step formed on the outer peripheral surface of the bottom portion of the permanent magnet piece without influencing the threaded portion of the plug main body, and owing to the fact that the hollow cylindrical piece is shrivelled against the permanent magnet piece over their entire circumference, the permanent magnet piece can be firmly and fixedly secured to the plug main body without producing any looseness therebetween.
Moreover, since the permanent magnet piece is fitted in and fixedly secured to the hollow cylindrical piece projected from the tip end surface of the threaded portion of the plug main body rather than being fitted within the threaded portion as is the case with the prior art, the threaded portion of the plug main body would not be magnetized by the permanent magnet piece, and therefore, it would never occur that a threaded bore in a gear box or the like for receiving the magnet plug as well as the threaded portion of the plug main body are damaged by iron powder adsorbed on the magnetized threaded portion of the plug main body.
The above-mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings.